Changing pulse width to reduce visible interference

ABSTRACT

In an optical system that includes a coherent light source and an optical waveguide, a pulse width used by the optical waveguide to project image frames on a display is changed on a frame-by-frame basis. By changing the pulse width for each image frame, the locations and characteristics of visible interference patterns on the display are changed for each successive image frame. Changing the interference patterns for each image frame may result in the interference patterns being less detectable to a viewer. The change in pulse width for each image frame may be fixed or dynamic, and may be made in response to interference patterns being detected on the display.

BACKGROUND

Optical waveguides can be used to expand or replicate the exit pupil ofan imaging system in one or two dimensions. Typically, light from theexit pupil of the imaging system is received in the waveguide through anentrance or in-coupling, and travels through the waveguide in adirection, while some of the light exits a grating structure of thewaveguide to a display or the eye of a user. The remaining light thatdoes not exit the grating structure may reflect off of the internalsurfaces of the waveguide before finally exiting through the gratingstructure or some other part of the waveguide.

One issue with current waveguide-based exit pupil expanders is they mayresult in visible interference when used with coherent light sources.Coherent light sources are light sources containing a narrow range offrequencies. Coherent light sources when split into multiple paths bythe waveguide will generally interfere with themselves when received bythe display if the difference in optical path length traversed by eachoptical path of the light is less than a coherence length associatedwith the coherent light source. An example of a coherent light source isa laser.

Coherent light sources have several advantages over current LCoS (liquidcrystal on silicon) based imaging systems, especially when used in headmounted display systems. For example, they have high sequential contrastratios, lower weight and size, and can be adjusted to compensate fornon-uniformities of the waveguide. Accordingly, there is a need toreduce the visible interference associated with coherent light sourcesand optical waveguides.

SUMMARY

In an optical system that includes a coherent light source and anoptical waveguide, a pulse width used by the optical waveguide toproject image frames on a display is changed on a frame-by-frame basis.By changing the pulse width for each image frame, the locations andcharacteristics of visible interference patterns on the display arechanged for each successive image frame. Changing the interferencepatterns for each image frame may result in the interference patternsbeing less detectable to a viewer. The change in pulse width for eachimage frame may be fixed or dynamic, and may be made in response tointerference patterns being detected on the display. Where the systemincludes multiple coherent light sources, the pulse widths used by eachof the coherent light sources may be similarly changed. The opticalsystem may be incorporated into a head mounted display device or othertype of display device that uses coherent light sources.

In an implementation, a system for reducing visible interferenceassociated with coherent light sources is provided. The system includesan optical waveguide, a coherent light source, and an interferenceengine. The interference engine is configured to receive a first imageframe, cause the coherent light source to project the first image frameinto the optical waveguide using a first pulse width, receive a secondimage frame, and cause the coherent light source to project the secondimage frame into the optical waveguide using a second pulse width,wherein the first pulse width is different than the second pulse width.

In an implementation, a method for reducing visible interferenceassociated with coherent light sources is provided. The method mayinclude receiving a first image frame at a computing device, selecting afirst pulse width by the computing device, projecting the first imageframe using the first pulse width by the computing device, receiving asecond image frame at the computing device, selecting a second pulsewidth by the computing device, wherein the second pulse width isdifferent than the first pulse width, and projecting the second imageframe using the second pulse width by the computing device.

In an implementation, a system for reducing visible interferenceassociated with coherent light sources is provided. The system mayinclude an optical waveguide, a first coherent light source, a secondcoherent light source, and an interference engine. The interferenceengine may be configured to: select a first pulse width and a secondpulse width, wherein the first pulse width is different than the secondpulse width; receive a first image frame; cause the first coherent lightsource to project a first portion of the first image frame into theoptical waveguide using the first pulse width; cause the second coherentlight source to project a second portion of the first image frame intothe optical waveguide using the second pulse width; receive a secondimage frame; cause the first coherent light source to project a firstportion of the second image frame into the optical waveguide using thesecond pulse width; and cause the second coherent light source toproject a second portion of the second image frame into the opticalwaveguide using the first pulse width.

This summary is provided to introduce a selection of concepts in asimplified form that are further described below in the detaileddescription. This summary is not intended to identify key features oressential features of the claimed subject matter, nor is it intended tobe used to limit the scope of the claimed subject matter.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing summary, as well as the following detailed description ofillustrative embodiments, is better understood when read in conjunctionwith the appended drawings. For the purpose of illustrating theembodiments, there is shown in the drawings example constructions of theembodiments; however, the embodiments are not limited to the specificmethods and instrumentalities disclosed. In the drawings:

FIG. 1 is an illustration of an exemplary head mounted display device;

FIG. 2 is an illustration of an exemplary near-eye display system;

FIG. 3 is an illustration of an exemplary interference pattern;

FIG. 4 is an illustration of an exemplary interference engine;

FIG. 5 is an operational flow of an implementation of a method forreducing visible interference in an HMD device using one or morecoherent light sources;

FIG. 6 is an operational flow of an implementation of a method forreducing visible interference in an HMD device using one or morecoherent light sources; and

FIG. 7 is an operational flow of an implementation of a method forreducing visible interference in an HMD device using two or morecoherent light sources.

DETAILED DESCRIPTION

FIG. 1 is an illustration of an example head mounted display (“HMD”)device 100. In an implementation, the HMD device 100 is a pair ofglasses. The HMD device 100 includes lenses 105 a and 105 b arrangedwithin a frame 109. The frame 109 is connected to a pair of temples 107a and 107 b. Arranged between each of the lenses 105 and a wearer's eyesis a near-eye display system 110. The system 110 a is arranged in frontof a right eye and behind the lens 105 a. The system 110 a is arrangedin front of a left eye and behind the lens 105 a. The HMD device 100also includes a controller 120 and one or more sensors 130. Thecontroller 120 may be a microcomputer operatively coupled to bothnear-eye display systems 110 a and 110 b and to the sensors 130. Othertypes of controllers 120 may be used.

The sensors 130 may be arranged in any suitable location on the HMDdevice 100. They may include a gyroscope or other inertial sensors, aglobal-positioning system (GPS) receiver, and/or a barometric pressuresensor configured for altimetry. The sensors 130 may provide data on thewearer's location or orientation. From the integrated responses of thesensors 130, the controller 120 may track the movement of the HMD device100 within the wearer's environment.

In some implementations, the sensors 130 may include an eye tracker thatis configured to detect an ocular state of the wearer of the HMD device100. The eye tracker may locate a line of sight of the wearer, measurean extent of iris closure, etc. If two eye trackers are included, onefor each eye, then the two may be used together to determine thewearer's focal plane based on the point of convergence of the lines ofsight of the wearer's left and right eyes. This information may be usedby the controller 120 for placement of a computer-generated image frame.The image frame may be a frame of a video, or the output of a computerapplication such as a video game, for example.

In some implementations, each of the near-eye display systems 110 a and110 b may be at least partly transparent, to provide a substantiallyunobstructed field of view in which the wearer can directly observetheir physical surroundings. Each of the near-eye display systems 110 aand 110 b may be configured to present, in the same field of view, acomputer-generated image frame.

The controller 120 may control the internal componentry of the near-eyedisplay systems 110 a and 110 b to form the desired image frame. In animplementation, the controller 120 may cause the near-eye displaysystems 110 a and 110 b to display approximately the same image frameconcurrently, so that the wearer's right and left eyes receive the sameimage frame at approximately the same time. In other implementations,the near-eye display systems 110 a and 110 b may project somewhatdifferent image frames concurrently, so that the wearer perceives astereoscopic, i.e., three-dimensional, image frame.

In some implementations, the computer-generated image frames and variousreal images of objects sighted through the near-eye display systems 110a and 110 b may occupy different focal planes. Accordingly, the wearerobserving a real-world object may shift their corneal focus to resolvethe image frame. In other implementations, the image frame and at leastone real image may share a common focal plane.

In the HMD device 100, each of the near-eye display systems 110 a and110 b may also be configured to acquire video of the surroundingssighted by the wearer. The video may include depth video and may be usedto establish the wearer's location, what the wearer sees, etc. The videoacquired by each near-eye display system 110 a, 110 b may be received bythe controller 120, and the controller 120 may be configured to processthe video received. To this end, the HMD device 100 may include acamera. The optical axis of the camera may be aligned parallel to a lineof sight of the wearer of the HMD device 100, such that the cameraacquires video of the external imagery sighted by the wearer. As the HMDdevice 100 may include two near-eye display systems—one for each eye—itmay also include two cameras. More generally, the nature and number ofthe cameras may differ in the various embodiments of this disclosure.One or more cameras may be configured to provide video from which atime-resolved sequence of three-dimensional depth maps is obtained viadownstream processing.

No aspect of FIG. 1 is intended to be limiting in any sense, fornumerous variants are contemplated as well. In some embodiments, forexample, a vision system separate from the HMD device 100 may be used toacquire video of what the wearer sees. In some embodiments, a singlenear-eye display system extending over both eyes may be used instead ofthe dual monocular near-eye display systems 110 a and 110 b shown inFIG. 1.

The HMD device 100 may be used to support a virtual-reality (“VR”) oraugmented-reality (“AR”) environment for one or more participants. Arealistic AR experience may be achieved with each AR participant viewingtheir environment naturally, through passive optics of the HMD device100. Computer-generated imagery may be projected into the same field ofview in which the real-world imagery is received. Imagery from bothsources may appear to share the same physical space.

The controller 120 in the HMD device 100 may be configured to run one ormore computer programs that support the VR or AR environment. In someimplementations, one or more computer programs may run on the controller120 of the HMD device 100, and others may run on an external computeraccessible to the HMD device 100 via one or more wired or wirelesscommunication links. Accordingly, the HMD device 100 may includesuitable wireless componentry, such as Wi-Fi.

FIG. 2 is an illustration of an exemplary near-eye display system 200.The near-eye display system 200 may be an implementation of one or bothof the near-eye display systems 110 a and 110 b shown in FIG. 1. In theexample shown, the system 200 includes a projector 290, an opticalwaveguide 250, a controller 120, and a display 280.

The projector 290 may be adapted to form an image frame, and to projectthe image frame through an exit pupil of the projector 290. Lightcorresponding to the image frame is shown in the near-eye display system200 as the light 209. The projector 290 may be operatively coupled tothe controller 120. The controller 120 may provide suitable controlsignals that, when received by the projector 290, cause the desiredimage frame to be formed.

The optical waveguide 250 may include a plurality of surfaces includinga front surface 205, a rear surface 206, a top surface 203, a bottomsurface 204, a left-side surface 201, and a right-side surface 202. Theoptical waveguide 250 may be substantially transparent to light receivednormal to the front surface 205 in the Z direction. Light receivednormal to the front surface 205 may pass through the front surface 205and the rear surface 206 to the display 280.

The optical waveguide 250 further includes an optical element 210. Theoptical element (“OE”) 210 may receive the light 209 from an exit pupilof the projector 290, and may cause a portion of the light 209 to enterthe optical waveguide 250 (i.e., in-couple). The portion of the light209 that enters the optical waveguide 250 is illustrated in the opticalwaveguide 250 as the light 215. Depending on the implementation, the OE210 may be a diffractive OE such as a diffractive grating. Examples ofsuitable diffractive gratings include surface-relief diffractiongratings (“SRGs”) or volumetric gratings. However, other types of OEsmay be used, such as mirrors and prisms, for example.

The OE 210 may cause the light 215 to propagate in the X directiontowards the right-side surface 202. In particular, the OE 210 may causethe light 215 to propagate in the X direction by reflecting off ofinterior surfaces of the optical waveguide 250.

The optical waveguide 250 may further include an OE 211. As the light215 propagates through the optical waveguide 250 and passes through theOE 211, the OE 211 may allow at least a portion of the light 215 to exitthe optical waveguide 250 (i.e., out-couple) through the rear surface206 as the light 225. Each ray of the light 225 may leave the rearsurface through an exit pupil of the optical waveguide 250. The light225 may be received by the display 280. Like the OE 210, the OE 211 maybe a diffractive coating on the front surface 205 such as an SRG. Othertypes of diffractive gratings may be used. While shown on the frontsurface 205, the OE 211 may also be applied to the rear surface 206. Thenumber and locations of the exit pupils of the optical waveguide 250 maydepend on the properties of the OE 211.

The portion of the light 215 that does not exit the optical waveguide250 through an exit pupil may continue in the X direction towards theright-side surface 202, The light 215 may exit the optical waveguide 250through the right-side surface 202, or may be internally reflected backthrough the optical waveguide 250 towards the left-side surface 201. Theinternally reflected light may then be out-coupled by the OE 211, or maycontinue to be internally reflected.

As may be appreciated, the light 225 exiting the optical waveguide 250through the exit pupils of the OE 211 is a pupil expansion of the exitpupil of the projector 290. Each arrow representing the light 225 mayexit through an exit pupil of the optical waveguide 250.

Typically, the projectors 290 use incoherent light sources, rather thancoherent light sources, to generate the light 209 that is used toproject the image frame. A coherent light source may be a light sourcewhose output light 209 includes photons that are oscillating in the samedirection. This is in contrast with incoherent light sources where thephotons may be oscillating in different directions. Examples of coherentlight sources include lasers, and examples of incoherent light sourcesinclude LED light sources.

While coherent light sources have many advantages over incoherent lightsources including higher power output and efficiency, they have onemajor drawback in that they are susceptible to interference when thetotal optical path is less than a coherence length of the coherent lightsource. The length of an optical path of a ray of light is the totaldistance that the ray of light travels from the projector 290 until itis received by the display 280, and includes the total distance that theray travels within the optical waveguide 250 as the light 215, and thedistance that the light travels from the rear surface 206 to the eye 280as the light 225.

As described above, because the rays of the light 215 make differentnumbers of passes through the OE 211 before being out-coupled as thelight 225, there may be many different path lengths for the rays of thelight 225 received by the display 280. Depending on the differencesbetween the lengths of the rays of light 225, there may either beconstructive or destructive interference visible on the display 280.

For example, FIG. 3 shows an example interference pattern that may beassociated with coherent light sources. The image frame 300 a is anexample of an image frame that is projected on the display 280 thatincludes a circle 303, The circle 303 is shown with a uniform mediumgrey color and has no visible interference patterns.

The image frame 300 b is an example of an image frame of the same circle303 but with visible interference patterns. In the example shown, thecircle 303 includes several rings 305 (i.e., rings 305 a-d) that arevisibly darker than the color of the circle 303 shown in the image frame300 a. These rings 305 are areas of constructive interference due to therays of light being in-phase with each other. The circle also includesseveral rings 307 (i.e., rings 307 a-c) that are visibly lighter thanthe color of circle 303 shown in the image frame 300 a. These rings 307are areas of destructive interference due to the rays of light beingout-of-phase with each other. The type of interference patterns shown inthe image frame 300 b are known as Newton's rings.

Returning to FIG. 2, in order to reduce the visible interference due tocoherent light sources, the near-eye display system 200 includes aninterference engine 260 that effectively changes the output spectra ofthe coherent light source on a-frame-by-frame basis. Changing thespectral characteristics for each image frame changes the locationand/or appearance of the visible interference pattern for the imageframes. Because of the high frame used by the near-eye display system200 (e.g., 120 Hz), the resulting interference patterns become averagedout and less visible to the wearer of the near-eye display system 200.

As described further below, one way of changing the spectralcharacteristics of the coherent light source is through pulse widthmodulation. The interference engine 260 may periodically adjust orchange a pulse width used by the coherent light source. In particular,the interference engine 260 may change the pulse width used by thecoherent light source for each image frame in a sequence of imageframes. Changing the pulse width changes the “effective temperature” ofthe coherent light source which may change the spectral characteristicsof the coherent light source. By changing the pulse width for each imageframe, the location and other characteristics of the resultinginterference pattern that is visible to the wearer of the near-eyedisplay system 200 is also changed for each image frame.

Because the visible coherence pattern is changed for each image frame,the overall amount of interference that is visible to the wearer of theHMD device 100 is reduced.

Other methods for changing the spectral characteristics of the coherentlight source may be used. For example, the interference engine 260 maychange the spectral characteristics by changing the pulse shape used bythe coherent light source. In another example, the interference engine260 may change the spectral characteristics by shifting the spectrumassociated with the coherent light source. Other methods may be usedsuch as a biasing scheme or by changing a laser cavity optical length,for example.

FIG. 4 is an illustration of an exemplary interference engine 260. Theinterference engine 260 includes components such as a spectral modifier405 and an interference detector 409. More or fewer components may besupported. The interference engine 260 may be implemented by one or morecomputing devices such as the HMD device 100.

The spectral modifier 405 may effectively change the spectralcharacteristics of a coherent light source 410 to reduce visibleinference with respect to an image frame 415 projected on a display 280.The coherent light source 410 may be a laser and may be part of aprojector 290. In some implementations, the spectral modifier 405 maychange the spectral characteristics of the coherent light source 410 foreach image frame 415. For example, the spectral modifier 405 may usefirst spectral characteristics for a first image frame 415, and may usesecond spectral characteristics for a second image frame 415. Thespectral modifier 405 may use the first spectral characteristics for athird image frame 415, or may use different spectral characteristics.

The spectral modifier 405 may change the spectral characteristics of thecoherent light source 410 by changing a pulse width used by the coherentlight source 410. Depending on the implementation, the spectral modifier405 may increase or decrease the pulse width of the coherence lightsource 410 used for successive image frames 415 by approximately one toten nanoseconds. The amount that the pulse width is changed may be fixedor randomly selected. The amount that the pulse width is changed may setby a user, or may be automatically selected during a calibration phase.Any method or technique for changing a pulse width of a coherent lightsource 410 may be used.

The spectral modifier 405 may further change the spectralcharacteristics by shifting a spectrum used by the coherent light source410. For example, the spectral modifier 405 may shift the spectrum ofthe coherent light source 410 by applying a bias current to the coherentlight source 410. The spectrum used by the coherent light source 410 maybe shifted by changing the amount of bias current that is applied. Othermethods for shifting a spectrum of a coherent light source 410 may beused.

The spectral modifier 405 may further change the spectralcharacteristics by changing a pulse shape used by the coherent lightsource 410. Any method for changing a pulse shape of a coherent lightsource 410 may be used.

In systems with more than one coherence light source 410, the spectralmodifier 405 may change the spectral characteristics of each of thecoherent light sources 410. For example, the projector 290 may include afirst coherent light source 410 that projects blue light, a secondcoherent light source 410 that projects red light, and third coherentlight source 410 that projects green light. For a first image frame 415,the spectral modifier 405 may cause the first coherent light source 410to project a portion of the first image frame 415 that corresponds tothe blue light using a first pulse width, may cause the second coherentlight source 410 to project a portion of the first image frame 415 thatcorresponds to the red light using the first pulse width, and may causethe third coherent light source 410 to project a portion of the firstimage frame 415 that corresponds to the green light using the firstpulse width.

For a subsequent second image frame 415, the spectral modifier 405 maychange the pulse widths used for each of the first, second, and thirdcoherent light sources 410. The spectral modifier 405 may use the samepulse widths for each coherent light source 410, may change the eachpulse width by a same amount, or may use a different pulse width foreach coherent light source 410. For example, the spectral modifier 405may cause the first coherent light source 410 to project a portion ofthe second image frame 415 that corresponds to the blue light using asecond pulse width, may cause the second coherent light source 410 toproject a portion of the second image frame 415 that corresponds to thered light using the second pulse width, and may cause the third coherentlight source 410 to project a portion of the second image frame 415 thatcorresponds to the green light using the second pulse width.

In some implementations, the projector 290 may include multiple coherentlight sources 410 per light color. For example, the projector 290 mayinclude two coherent light sources 410 for the color red, two coherentlight sources 410 for the color blue, and two coherent light sources 410for the color green. In these implementations, for each light color, thespectral modifier 405 may cause the first coherent light source 410 touse a first pulse width and the second coherent light source 410 to usea second pulse width for a first image frame 415, and may cause thefirst coherent light source 410 to use the second pulse width and thesecond coherent light source 410 to use the first pulse width for asubsequent second image frame 415. Other combinations of pulse widthsmay be used.

The interference detector 409 may detect or measure interference in aprojected image frame 415, and may instruct the spectral modifier 405 tochange the spectral characteristics of one or more coherent lightsources 410 in response to the determination. In some implementations,the interference detector 409 may operate as part of a calibration phasewhere various image frames 415 are projected onto the display 280 by thecoherent light sources 410. The performance of the coherent lightsources 410 may be measured for various metrics such as brightness,contrast level, sharpness, and any interference patterns may bedetected. Any method or technique for detecting interference patternsmay be used.

The interference detector 409 may instruct the spectral modifier 405 toadjust the spectral characteristics in response to any detectedinterference patterns. For example, the interference detector 409 mayinstruct the spectral modifier 405 to change the pulse width used by thecoherent light source 410 by some amount for each successive image frame415. After making the change, the interference detector 409 may continueto measure the metrics associated with the display 280 to ensure thatthe change in the spectral characteristics has not resulted in anyadverse effects such as a reduction in brightness. If the change in thepulse width results in adverse effects, the interference detector 409may recommend a smaller change in the pulse width be used for each imageframe 415, for example.

In some implementations, the interference detector 409 may continuouslymonitor the image frames 415 projected on the display 280 forinterference. In response to the detected interference, the interferencedetector 409 may instruct the spectral modifier 405 to change the pulsewidth used by the coherent light source 410. Alternatively oradditionally, the interference detector 409 may instruct the spectralmodifier 405 to change the pulse shape or shift the spectrum used by thecoherent light source 410.

FIG. 5 is an operational flow of an implementation of a method 500 forreducing visible interference in an HMD device 100 using one or morecoherent light sources 410. The method 500 may be implemented by theinterference engine 260, for example.

At 501, a first image frame is received. The first image frame 415 maybe received by the coherent light source 410. The coherent light source410 may be a laser. The first image frame 415 may be an image frame 415of a video or output of an application currently being viewed and/orused by a wearer of an HMD device 100. The HMD device 100 may include anoptical waveguide 250 and a display 280.

At 503, the coherent light source is caused to project the first imageframe into the optical waveguide using a first pulse width. The coherentlight source 410 may be caused to project the first image frame 415using the first pulse width by the interference engine 260.

At 505, a second image frame is received. The second image frame 415 maybe received by the coherent light source 410. The second image frame 415may be a next image frame 415 of the video or the output of theapplication currently being viewed and/or used by the wearer of the HMDdevice 100.

At 507, the coherent light source is caused to project the second imageframe into the optical waveguide using a second pulse width. Thecoherent light source 410 may be caused to project the second imageframe 415 using the second pulse width by the interference engine 260.The second pulse width may be selected by the interference engine 260 toreduce visible interference on the display 280. A difference between thefirst pulse width and the second pulse width may be between one and tennanoseconds. Depending on the implementation, additional steps may betaken to reduce visible interference such as shifting a spectrumassociated with the coherent light source 410 or changing a pulse widthshape.

At 509, a third image frame is received. The third image frame 415 maybe received by the coherent light source 410. The third image frame 415may be a next image frame 415 of the video or the output of theapplication currently being viewed and/or used by the wearer of the HMDdevice 100.

At 511, the coherent light source is caused to project the third imageframe into the optical waveguide using the first pulse width. Thecoherent light source 410 may be caused to project the third image frame415 using the first pulse width by the interference engine 260.Alternatively, the coherent light source 410 may be caused to projectthe third image frame 415 using a third pulse width.

FIG. 6 is an operational flow of an implementation of a method 600 forreducing visible interference in an HMD device 100 using one or morecoherent light sources 410. The method 600 may be implemented by theinterference engine 260, for example.

At 601, a first image frame is received. The first image frame 415 maybe received by the coherent light source 410. The coherent light source410 may be a laser. The first image frame 415 may be an image frame 415of a video or output of an application currently being viewed and/orused by a wearer of an HMD device 100. The HMD device 100 may include anoptical waveguide 250 and a display 280.

At 603, a first pulse width is selected. The first pulse width may beselected by the spectral modifier 405.

At 605, the first image frame is projected using the first pulse width.The first image frame 415 may be projected by the coherent light source410 into the optical waveguide 250.

At 607, interference is detected. The interference may be visibleinterference that is detected by the interference detector 409 in theprojected first image frame 415. The visible interference may beassociated with the use of the coherent light source 410. Any method fordetecting visible interference may be used. Depending on theimplementation, the interference may be detected as part of acalibration phase associated with the HMD device 100.

At 609, a second image frame is received. The second image frame 415 maybe received by the coherent light source 410. The second image frame 415may be a next image frame of the video or the output of the applicationcurrently being viewed and/or used by the wearer of the HMD device 100.

At 611, a second pulse width is selected. The second pulse width may beselected by the spectral modifier 405. The second pulse width may beselected based on the detected interference to effectively change thespectral characteristics of the coherent light source 410. By changingthe spectral characteristics the location and characteristics of thedetected interference may be changed resulting in the interference beingoverall less visible to the user. Other methods for changing thespectral characteristics of the coherent light source 410 may be used,including shifting a spectrum of the coherent light source 410 andchanging a pulse shape of the coherent light source 410.

At 613, the second image from is projected using the second pulse width.The second image frame 415 may be projected by the coherent light source410 into the optical waveguide 250.

FIG. 7 is an operational flow of an implementation of a method 700 forreducing visible interference in an HMD device 100 using two or morecoherent light sources 410. The method 700 may be implemented by theinterference engine 260, for example.

At 701, a first pulse width and a second pulse width are received. Thefirst pulse width and the second pulse width may be received by theprojector 290 from the spectral modifier 405. The first and second pulsewidths may have been selected to reduce visible interference in the HMDdevice 100 without unduly harming characteristics of the HMD device 100such as brightness and contrast. Depending on the implementation, adifference between the first pulse width and the second pulse width maybe between one and ten nanoseconds.

The first and second pulse widths may be received for a first coherentlight source 410 and a second coherent light source 410. The firstcoherent light source 410 and the second coherent light source 410 maybe associated with the same or different color of light. The firstcoherent light source 410 and the second coherent light source 410 maybe lasers.

At 703, a first image frame is received. The first image frame 415 maybe received by the first coherent light source 410 and the secondcoherent light source 410. The first image frame 415 may have a firstportion and a second portion. In implementations where the firstcoherent light source 410 and the second coherent light source 410 arethe same color of light, the first portion of the first image frame 415may correspond to even lines of the first image frame 415 and the secondportion of the first image frame 415 may correspond to odd lines of thefirst image frame 415. In implementations where the first coherent lightsource 410 and the second coherent light source 410 correspond todifferent colors, the first portion of the first image frame 415 maycorrespond to blue colors of the first image frame 415 and the secondportion of the first image frame 415 may correspond to red colors of thefirst image frame 415. Other configurations and colors may be supported.

At 705, the first coherent light source is caused to project the firstportion of the first image frame using the first pulse width. The firstcoherent light source 410 may be caused to project the first portion ofthe first image frame 415 using the first pulse width by theinterference engine 260. The first portion of the first image frame 415may be projected into an optical waveguide 250 of the HMD device 100.

At 707, the second coherent light source is caused to project the secondportion of the first image frame using the second pulse width. Thesecond coherent light source 410 may be caused to project the secondportion of the first image frame 415 using the second pulse width by theinterference engine 260. The second portion of the first image frame 415may be projected into the optical waveguide 250 of the HMD device 100.

At 709, a second image frame is received. The second image frame 415 maybe received by the first coherent light source 410 and the secondcoherent light source 410. The second image frame 415 may have a firstportion and a second portion.

At 711, the first coherent light source is caused to project the firstportion of the second image frame using the second pulse width. Thefirst coherent light source 410 may be caused to project the firstportion of the second image frame 415 using the second pulse width bythe interference engine 260.

At 713, the second coherent light source is caused to project the secondportion of the second image frame using the first pulse width. Thesecond coherent light source 410 may be caused to project the secondportion of the second image frame 415 using the first pulse width by theinterference engine 260.

It should be understood that the various techniques described herein maybe implemented in connection with hardware components or softwarecomponents or, where appropriate, with a combination of both.Illustrative types of hardware components that can be used includeField-programmable Gate Arrays (FPGAs), Application-specific IntegratedCircuits (ASICs), Application-specific Standard Products (ASSPs),System-on-a-chip systems (SOCs), Complex Programmable Logic Devices(CPLDs), etc. The methods and apparatus of the presently disclosedsubject matter, or certain aspects or portions thereof, may take theform of program code (i.e., instructions) embodied in tangible media,such as floppy diskettes, CD-ROMs, hard drives, or any othermachine-readable storage medium where, when the program code is loadedinto and executed by a machine, such as a computer, the machine becomesan apparatus for practicing the presently disclosed subject matter.

In an implementation, a system for reducing visible interferenceassociated with coherent light sources is provided. The system includes:an optical waveguide; a coherent light source; and an interferenceengine. The interference engine is configured to: receive a first imageframe; cause the coherent light source to project the first image frameinto the optical waveguide using a first pulse width; receive a secondimage frame; and cause the coherent light source to project the secondimage frame into the optical waveguide using a second pulse width,wherein the first pulse width is different than the second pulse width.

Implementations may include some or all of the following features. Theinterference engine may be further configured to select a differencebetween the first pulse width and the second pulse width. The coherentlight source may be a laser. The system may be part of a head mounteddisplay device. The interference engine may be further configured todetect interference in the projected first image frame, and in responseto the detection, cause the coherent light source to project the secondimage frame into the optical waveguide using the second pulse width. Theinterference engine may be further configured to: receive a third imageframe; and cause the coherent light source to project the third imageframe into the optical waveguide using the first pulse width.

In an implementation, a method for reducing visible interferenceassociated with coherent light sources is provided. The method mayinclude: receiving a first image frame at a computing device; selectinga first pulse width by the computing device; projecting the first imageframe using the first pulse width by the computing device; receiving asecond image frame at the computing device; selecting a second pulsewidth by the computing device, wherein the second pulse width isdifferent than the first pulse width; and projecting the second imageframe using the second pulse width by the computing device.

Implementations may include some or all of the following features. Themethod may include detecting interference in the first image frame, andselecting the second pulse width based on the detected interference. Thecomputing device may include a coherent light source, and projecting thefirst image frame using the first pulse width by the computing devicemay include projecting the first image frame using the first pulse widthby the coherent light source. The method may further include shifting aspectrum associated with the coherent light source. The coherent lightsource may be a laser. The computing device may include a first coherentlight source and a second coherent light source, and projecting thefirst image frame using the first pulse width may include: projecting afirst portion of the first image frame by the first coherent lightsource using the first pulse width; and projecting a second portion ofthe first image frame by the second coherent light source using thefirst pulse width. Projecting the second image frame using the secondpulse width may include: projecting a first portion of the second imageframe by the first coherent light source using the second pulse width;and projecting a second portion of the second image frame by the secondcoherent light source using the second pulse width. The computing devicemay be part of a head mounted display device. The method may include:receiving a third image frame at the computing device; selecting thefirst pulse width by the computing device; and projecting the thirdimage frame using the first pulse width by the computing device.

In an implementation, a system for reducing visible interferenceassociated with coherent light sources is provided. The system mayinclude: an optical waveguide; a first coherent light source; a secondcoherent light source; and an interference engine. The interferenceengine may be configured to: select a first pulse width and a secondpulse width, wherein the first pulse width is different than the secondpulse width; receive a first image frame; cause the first coherent lightsource to project a first portion of the first image frame into theoptical waveguide using the first pulse width; cause the second coherentlight source to project a second portion of the first image frame intothe optical waveguide using the second pulse width; receive a secondimage frame; cause the first coherent light source to project a firstportion of the second image frame into the optical waveguide using thesecond pulse width; and cause the second coherent light source toproject a second portion of the second image frame into the opticalwaveguide using the first pulse width.

Implementations may include some or all of the following features. Theinterference engine may be further configured to: receive a third imageframe; cause the first coherent light source to project a first portionof the third image frame into the optical waveguide using the firstpulse width; and cause the second coherent light source to project asecond portion of the third image frame into the optical waveguide usingthe second pulse width. The system may be part of a head mounted displaydevice. The first coherent light source may be a laser. The firstcoherent light source may be associated with a first color of light, andthe second coherent light source may be associated with a second colorof light.

Although the subject matter has been described in language specific tostructural features and/or methodological acts, it is to be understoodthat the subject matter defined in the appended claims is notnecessarily limited to the specific features or acts described above.Rather, the specific features and acts described above are disclosed asexample forms of implementing the claims.

1. A system for reducing visible interference associated with coherentlight sources, the system comprising: an optical waveguide; a coherentlight source; and an interference engine configured to: receive a firstimage frame; cause the coherent light source to project the first imageframe into the optical waveguide using a first pulse width; detecting aninterference pattern in the first image frame; receive a second imageframe; and cause the coherent light source to project the second imageframe into the optical waveguide using a second pulse width, wherein thefirst pulse width is different than the second pulse width and isselected based on the interference pattern detected in the first image.2. The system of claim 1, wherein the interference engine is furtherconfigured to select a difference between the first pulse width and thesecond pulse width.
 3. The system of claim 1, wherein the coherent lightsource comprises a laser.
 4. The system of claim 1, wherein the systemis part of a head mounted display device.
 5. The system of claim 1,wherein the interference engine is further configured to detectinterference in the projected first image frame, and in response to thedetection, cause the coherent light source to change pulse width used bythe coherent light source by a predetermined amount for each successiveimage frame.
 6. The system of claim 1, wherein the interference engineis further configured to: receive a third image frame; and cause thecoherent light source to project the third image frame into the opticalwaveguide using the first pulse width.
 7. A method for reducing visibleinterference associated with coherent light sources, the methodcomprising: receiving a first image frame at a computing device;selecting a first pulse width by the computing device; projecting thefirst image frame using the first pulse width by the computing device;receiving a second image frame at the computing device; selecting asecond pulse width by the computing device, wherein the second pulsewidth is different than the first pulse width and is selected based onan interference pattern in the first image; and projecting the secondimage frame using the second pulse width by the computing device.
 8. Themethod of claim 7, further comprising detecting interference in thefirst image frame, and in response to the detection, causing thecoherent light source to change pulse width used by the coherent lightsource by a predetermined amount for each successive image frame.
 9. Themethod of claim 7, wherein the computing device comprises a coherentlight source, and projecting the first image frame using the first pulsewidth by the computing device comprises projecting the first image frameusing the first pulse width by the coherent light source.
 10. The methodof claim 9, further comprising shifting a spectrum associated with thecoherent light source.
 11. The method of claim 9, further comprisingchanging a pulse shape associated with the coherent light source. 12.The method of claim 7, wherein the computing device comprises a firstcoherent light source and a second coherent light source, and projectingthe first image frame using the first pulse width comprises: projectinga first portion of the first image frame by the first coherent lightsource using the first pulse width; and projecting a second portion ofthe first image frame by the second coherent light source using thefirst pulse width.
 13. The method of claim 12, wherein projecting thesecond image frame using the second pulse width comprises: projecting afirst portion of the second image frame by the first coherent lightsource using the second pulse width; and projecting a second portion ofthe second image frame by the second coherent light source using thesecond pulse width.
 14. The method of claim 12, wherein the computingdevice is part of a head mounted display device.
 15. The method of claim7, further comprising: receiving a third image frame at the computingdevice; selecting the first pulse width by the computing device; andprojecting the third image frame using the first pulse width by thecomputing device.
 16. A system for reducing visible interferenceassociated with coherent light sources, the system comprising: anoptical waveguide; a first coherent light source; a second coherentlight source; and an interference engine configured to: select a firstpulse width; receive a first image frame; cause the first coherent lightsource to project a first portion of the first image frame into theoptical waveguide using the first pulse width; detect an interferencepattern in the first image frame select a second pulse width based on aninterference pattern, the second pulse width being different than thefirst pulse width; cause the second coherent light source to project asecond portion of the first image frame into the optical waveguide usingthe second pulse width; receive a second image frame; cause the firstcoherent light source to project a first portion of the second imageframe into the optical waveguide using the second pulse width; and causethe second coherent light source to project a second portion of thesecond image frame into the optical waveguide using the first pulsewidth.
 17. The system of claim 16, wherein the interference engine isfurther configured to: receive a third image frame; cause the firstcoherent light source to project a first portion of the third imageframe into the optical waveguide using the first pulse width; and causethe second coherent light source to project a second portion of thethird image frame into the optical waveguide using the second pulsewidth.
 18. The system of claim 16, wherein the system is part of a headmounted display device.
 19. The system of claim 16, wherein the firstcoherent light source comprises a laser.
 20. The system of claim 16,wherein the first coherent light source is associated with a first colorof light, and the second coherent light source is associated with asecond color of light.